In one save I killed him in his dragon form for some sick loot. and need to know how to change back to human form from dragon form.

Slane is his name, and he's located in an eastern part of the Hollow Marshes. Some fights are just too swarming and sometimes I wanna feel powerful so thank you for making the mod. You obtain a stone after conquering Laiken and taking control of the Battle Tower on Sentinel Island with some help from Sassan. You get to change into a Dragon for the last open area, but this open area is easily 2/3rds of the game. All trademarks are property of their respective owners in the US and other countries. You must be in dragon form to access the dragon armor and dragon skill tabs. Take your favorite fandoms with you and never miss a beat. I'm playing on XBox 360 (so glad I found this game!) © Valve Corporation.

This leads me to believe it's about as simple as can be, and I just don't see it.